Cementitious boards useful in the construction industry are known to contain inorganic, hydraulically setting material, such as Portland cement or gypsum. Hydraulic gypsum and cement, once set, have very little tensile strength and are usually reinforced with facing materials which improve the resistance to tensile and flexural loads. This has been the basis for using paper facing on conventional gypsum wall board and nonwoven glass fiber scrim in cement boards.
Glass fiber meshes have been popular as a facing sheet in cement boards because they can increase the dimensional stability in the presence of moisture and provide greater physical and mechanical properties. However, most glass fiber compositions, other than AR glass, degrade in the alkali environment of a cement core, so they must be coated with a protective finish.
Cementitious boards have been manufactured by casting a hydraulic cement mixture in the form of a thin, indefinitely long panel. See U.S. Pat. No. 4,504,335, which is hereby incorporated by reference. The hydraulic cement is usually a mortar containing a mixture of water and Portland cement sand, mineral or non-mineral aggregate, fly ash, accelerators, plasticizers, foaming agents and/or other additives. The mortar slurry is deposited onto a glass reinforcing network having a strippable paper sheet thereon, which is fed from a roll to pass over the forming table and under a continuous stream of mortar. The mortar is then distributed across the breadth of the carrier sheet, and the mortar-laden carrier sheet is towed through a slit defined by a supporting surface and a cylindrical mortar screeding roller mounted above the supporting surface so that its axis is transversely parallel to the supporting surface. The long network of reinforcing fibers is drawn against the roller and through the slit, rotating the roller counter to the direction of the travel of the carrier sheet, whereby the roller presses the network into the surface of the mortar and wipes mortar adhering to the roller into the interstices of the network. The network then tows the resulting broad, flat ribbon of mortar towards a cutter.
Similarly, British Patent Specification No. 772,581 teaches a production of reinforced plasterboard by a method which comprises spreading plaster on a first conveyor belt, dumping the plaster onto a plaster-soaked reinforcing mesh which is being transported by a second conveyor belt, and passing the plaster under a pressure roller to produce a ribbon of the required thickness. A second plaster-soaked mesh is dragged onto the upper surface of the ribbon as the mesh is fed under a third conveyor belt mounted above and in pressing relationship to the ribbon of plaster.
In still another process, as described in Lehnert et al., U.S. Pat. No. 4,647,496, a randomly oriented fibrous glass mat is fed onto a continuously moving belt onto which gypsum slurry is poured. The top surface of the gypsum ribbon thus formed is layered with a second randomly oriented glass mat which forms a sandwich with the gypsum core and the lower glass mat.
Woven knit and laid scrim fabrics may be coated either:
(a) before fabric-forming, as in single-end-coated fabrics;
(b) in-line (normally roller or dip coated) concurrently with formation such as in the case of laid scrim nonwoven meshes; or
(c) off-line coated after formation (normally roller or dip coated), typically used with many woven fabrics. In the case of coating before fabric-forming, the cost of coating each strand individually, in an operation prior to weaving, may be prohibitive. In the cases of in-line or off-line coating operations, the coating levels of the MD and CD yarns are generally not independent.
When woven, knit or mesh-type (scrim) nonwoven fabrics are dip or roller coated with resinous materials for imparting strength, abrasion resistance, fire retardancy, pigmentation and other properties, absorbent multi-filament yarns or strands are often used to prepare the fabrics. When the input yarns are of significant twist (over 0.1 turns/inch), the twist affects the ratio of coating weight in the cross-machine direction (weft yarns) versus the coating weight in the machine direction (warp yarns). Generally, multi-filament yarn based fabrics collect less coating in the warp direction and more coating in the weft direction. This is due to the asymmetry of tensions in the two directions—the warp yarns normally have higher tension, which is necessary to pull the fabric through the coating, drying, winding processes. In this description a strand is a single bundle of filaments—either continuous filaments or staple filaments. A yarn is a strand with some integrity of the filaments in the bundle—typically achieved by twisting the strand. The wet pick-up or WPU of a strand or yarn is defined as the weight of liquid coating on a yarn or strand divided by the weight of the strand or yarn, expressed as a percentage. The WPU of a strand in a dipping or roll coating process is determined in part by the following relationship:WPUactual=WPUmax−K×tension×twist frequencywhere K is a “wetting parameter”, a constant, depending on the strand surface area, certain liquid properties of the coating and the filament properties. The tension is the load applied to the strand in an axial direction often expressed in Newtons or grams-force. The twist frequency is the rate at which the strand of filaments is twisted often expressed in turns per inch or turns per meter.
The weft or cross-machine direction strands of a substantially orthogonal (woven or laid scrim) fabric are normally under very low tension. The warp or machine direction strands or yarns are normally under higher tension to facilitate pulling the fabric through the coating process. In this case the coating weight distribution ratio (WPUcd/WPUmd)=f((WPUcd max−Kcd×tensioncd×twist frequencycd)/(WPUmd max−Kmd×tensionmd×twist frequencycd). Assuming that the twist and wetting parameters of the yarns in the machine and cross-machine direction remain the same with respect to each other, the higher MD and lower CD tension associated with processing typical woven, knitted and laid-scrim fabrics results in more coating being applied to the CD or cross-machine direction. Typical weight distribution ratios (WPUcd/WPUmd) are greater than 2.0:1 to about 3.0:1, and are usually 2.5:1.
A balanced coating weight distribution is desirable. It is easy to achieve in the case of single-end-coated (SEC) fabrics as each strand is independently and explicitly coated with a given level of coating. The coated strands are then combined into a fabric with the ratio of coating (DPUcd/DPUmd) being established simply by selection of yarns containing the desired coating weights-often selected to be the same in MD and CD.
Unequal coating levels between the MD and CD yarns, normally found in dip coated fabrics, an “imbalanced coating weight distribution ratio”, often leads to undesirable properties of reinforcements especially those which have been treated or coated for corrosion or fire resistance. In corrosive environments, such as cement-based matrices, heavier coating in the CD implies lower, possible inadequate coating protection on the MD. Both quantity and quality of coating in the MD suffers. The tensioned, twisted MD bundle does not allow coating to penetrate within the bundle. As a result substantial pockets of air remain in the MD bundle. The poor quantity and quality of coating on the MD strands leads to poor corrosion protection of said strands relative to that of the CD strands.
Accordingly, there remains a need for woven, knit or mesh-type non-woven (“scrim”) fabrics which have a more uniform coating, as well as methods for producing a uniform coating on such fabrics for improving aesthetic qualities and for protecting these fabrics in environments which require corrosion and flame resistance, for example.